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LUCIA AND ATBEVIDA GLACIERS 79
white area are highly inclined and in some cases vertical. All these facts harmonize with the hypothesis that at this point there was an upwelling in the glacier, bringing clear ice to the surface and pushing aside the debris-covered ice that previously stood here. We arenot able to postulate any other hypothesis which will explain these notable facts. The process which we conceive is, first, the fairly-free flowage of the ice down the mountain valley. Beyond the mountains further flowage was resisted by the stagnant, expanded bulb, which, being thin, was too rigid to easily yield to the pressure upon it. It yielded somewhat, however, and a series of crescentic gashes were opened in the piedmont area, but the resistance was so great that relief for the pressure from above was found in three other ways: (1) by an advance westward toward the most open area, (2) by a pronounced thickening and doming of t.Tn'p part of the glacier, and (S) by the uprising of ice from below. The future of this area of clear ice seems to promise the development of an interior flat—a nearly moraine-free depression in the midst of ablation moraine such as is observed in the Variegated, Galiano, and other glaciers. The cause for these interior flats has hitherto been a mystery to ua, but this hypothesis offers an explanation. Further consideration of tM" subject will be deferred to a later chapter after the characteristics of other, older, and better-developed interior flats have been studied. If this hypothesis is correct the phenomenon of interior flats must have an important bearing on the question of the mode of motion of glaciers. Many facts observed in this region of advancing glaciers point to viscous flowage as the mode of motion, but the discussion of this subject also will be deferred until other facts of observation have been stated.